There is provided herein a process for converting methane to hydrocarbons having at least two carbon atoms (i.e. C.sub.2 +hydrocarbons). The processes involves contacting methane with an oxidizing agent, such as oxygen, under conditions of high pressure and moderate temperature.
Natural gas is an abundant fossil fuel resource. Recent estimates places worldwide natural gas reserves at about 35.times.10.sup.14 standard cubic feet, corresponding to the energy equivalent of about 637 billion barrels of oil.
The composition of natural gas at the wellhead varies but the major hydrocarbon present is methane. For example the methane content of natural gas may vary within the range of from about 40 to 95 vol. %. Other constituents of natural gas may include ethane, propane, butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen.
Natural gas is classified as dry or wet depending upon the amount of condensable hydrocarbons contained in it. Condensable hydrocarbons generally comprise C.sub.3 +hydrocarbons although some ethane may be included. Gas conditioning is required to alter the composition of wellhead gas, processing facilities usually being located in or near the production fields. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
Processed natural gas, consisting essentially of methane, (typically 85-95 volume percent) may be directly used as clean burning gaseous fuel for industrial heat and power plants, for production of electricity, and to fire kilns in the cement and steel industries. It is also useful as a chemicals feedstock, but large-scale use for this purpose is largely limited to conversion to synthesis gas which in turn is used for the manufacture of methanol and ammonia. It is notable that for the foregoing uses no significant refining is required except for those instances in which the wellhead-produced gas is sour, i.e., it contains excessive amounts of hydrogen sulfide. Natural gas, however, has essentially no value as a portable fuel at the present time. In liquid form, it has a density of 0.415 and a boiling point of minus 162.degree. C. Thus, it is not readily adaptable to transport as a liquid except for marine transport in very large tanks with a low surface to volume ratio, in which unique instance the cargo itself acts as refrigerant, and the volatilized methane serves as fuel to power the transport vessel. Large-scale use of natural gas often requires a sophisticated and extensive pipeline system.
A significant portion of the known natural gas reserves is associated with fields found in remote, difficulty accessible regions. For many of these remote fields, pipelining to bring the gas to potential users is not economically feasible.
Indirectly converting methane to methanol by steam-reforming to produce synthesis gas as a first step, followed by catalytic synthesis of methanol is a well-known process. The Mobil Oil Process, developed in the last decade provides an effective means for catalytically converting methanol to gasoline, e.g. as described in U.S. Pat. No. 3,894,107 to Butter et al. Although the market for gasoline is huge compared with the market for methanol, and although this process is currently used in New Zealand, it is complex and its viability appears to be limited to situations in which the cost for supplying an alternate source of gasoline is exceptionally high. There evidently remains a need for other ways to convert natural gas to higher valued and/or more readily transportable products.
Oxidative coupling is a process for the direct conversion of methane to higher hydrocarbons. It is normally performed at 700.degree.-900.degree. C. and less than 10 atm. While successful lower temperature operation has been a sought after goal, it has been found that at temperatures below about 700.degree. C., the selectivity to C.sub.2 + products rapidly diminishes, and below about 650.degree. C. C.sub.2 + production is essentially nil.
A second type of direct upgrading process is partial oxidation of methane to useful oxygenate products (i.e. methanol or formaldehyde). This approach operates at much higher pressures (20-100 atm.), but at more moderate temperatures (350.degree.-450.degree. C.) than oxidative coupling.